The present invention relates to the field of flow measurement and specifically the sensing of flow in aggressive, ultra-pure chemicals such as those typically used in semiconductor manufacturing. The present invention allows for the determination of a fluid flow rate based on differential pressure caused by a flow restriction. The pressure signals are processed by a DSP based electronic circuit, quantified by a microprocessor, and communicated to the end-user via a PC based graphical user interface or other display. The differential pressure sensor is highly accurate owing to the dimensions of the flow restriction and pressure sensor cavities, and permits pressure measurement in the restriction region in order to generate a large differential pressure. This maximized pressure differential increases the sensitivity and accuracy of the final flow rate measurement. Also, the dimensions of these critical regions reduce the overall pressure loss due to the measurement, a further enhancement over existing designs and of substantial benefit when measuring aggressive, high-purity fluids at relatively low flow rates.
During the manufacture of semiconductors, many different fluids must be precisely and accurately dispensed and deposited on the substrate being treated, such as deionized water, photoresist, spin on dielectrics, polyimides, developer and chemical mechanical polishing (CMP) slurries, to name a few. For example, in conventional apparatus for such applications, wafers to be processed are positioned beneath a suitable nozzle that then dispenses a predetermined amount of liquid or slurry to coat or treat the wafer. The predetermined amount is premised on pump cycles, tubing diameters and other characteristics of the fluid containment environment, not on the absolute amount or mass of fluid deposited on the wafer. Typically the wafer is then rotated to disperse the deposited liquid evenly over the entire surface of the wafer. It is readily apparent that the rate of dispensing and the amount of liquid dispensed are critical in this process.
When fluid flow is stopped through the nozzle, such as between wafer treatments, the potential exists for droplets of liquid from the nozzle to form and fall onto the wafer positioned below the nozzle. This can destroy the pattern being formed on the wafer, requiring that the wafer be discarded or reprocessed. In order to avoid the formation of deleterious droplets on the nozzle, external suckback or stop/suckback valves are commonly used. The latter such valves are typically a dual pneumatically controlled valve pair, with one valve stopping the flow of liquid to the nozzle, and the other drawing the liquid back from the dispense end or outlet port of the nozzle. This not only helps prevent droplet formation and dripping at the port, but also helps prevent drying of the exposed surface of the liquid, which can lead to clogging of the nozzle, and reduces fluid contamination at the outlet.
The coating of larger wafers (e.g., 300 mm in diameter and larger) is also problematic, as turbulence issues arise. The rotational speed of the wafer is conventionally used to spread the coating fluid from the center of the wafer where it is applied, radially outwardly to the edge of the wafer. However, this approach creates turbulent air flow over the wafer and can result in uneven or nonuniform coatings. Reducing the spin speed with larger wafers reduces the turbulence at the surface of the wafer, but can introduce new problems. With the reduced speed, the fluid moves across the wafer more slowly, and thus spreading the fluid to the wafer edge before the fluid begins to setup or dry becomes an issue.
Pumps conventionally have been used to dispense liquids in semiconductor manufacturing operations. However, the pumps suitable for such applications are expensive and require frequent replacement due to excessive wear.
It therefore would be desirable to provide a valve system that results in precise, reproducible dispensing of fluid without the foregoing disadvantages. Such a valve system should not be affected by changes in fluid temperature or effects of upstream fluid pressure. In addition, the present invention has broader applications to any fluid control device, especially where precise control of fluid flow is desired or required.